1. Field of the Invention
This invention relates to systems in which an alternate power source provides uninterrupted power to a load when commercial power fails or becomes unreliable and more particularly the invention relates to such a system providing uninterrupted power to the load from the commercial source when the alternate power source is taken out of service such as for maintenance.
2. Background of the Invention
In many installations it is common to have an alternate ac power source in the event that commercial power is lost or becomes unreliable. Such an alternate power source often comprises for instance an ac generator driven by an internal combustion engine. For switching between sources, a transfer switch disconnects one power source and connects the other. Such a transfer switch typically provides an open, or break-before-make, transition between the two power sources. That is, the transfer switch disconnects the source initially supplying power to the load before connecting the second source. This avoids the surges that would occur if two unsynchronized sources were connected to the load simultaneously.
Usually the transfer switches are specially designed devices which provide open transition switching. In some instances, transfer switches have been designed utilizing a pair of circuit breakers and a mechanism for operating the two circuit breakers in opposition. U.S. Pat. No. 3,778,633 discloses such a device in which the two circuit breakers mounted end to end are operated in opposition by a ganging member driven rectilinearly by a screw shaft engaging a traveling nut connected to the ganging member. In another switching device of this type, the handles of two side-by-side circuit breakers are operated by clevices attached to a lever arm which is rotated about a pivot axis between the two circuit breakers. In another transfer switching device using two side by side circuit breakers, slides engaging the circuit breaker handles are driven in opposition by gears having an eccentric pin which engages a camming surface on the slide.
All of the aforementioned transfer switches provide open transition operation and thus, momentarily interrupt power to the load. Such an interruption of power is unacceptable in certain applications. For instance, digital computers with volatile memories lose all the stored information if power is disconnected for longer than a very short interval. Uninterruptible power supplies have been developed for such applications. One such uninterruptible power supply includes a battery which is normally charged by a battery charger connected to the commercial power system. When commercial power is lost, an inverter fed by the battery provides ac power to the load. A special static switch provides a rapid open transfer between the commercial power source and the inverter. The resultant interruption of power only lasts for a few milliseconds which is not of long enough duration to adversely affect loads such as computer memories.
At times it becomes necessary to remove the uninterruptible power supply from the system for maintenance.
It is a primary object of the invention to provide an electric power system in which an uninterruptible power source can be removed from the system for maintenance automatically without damaging interruption of power to a load.
It is a more specific object of the invention to provide such a system in which an automatic closed, that is a make-before-break transition is effected when removing the uninterrupted power supply and switching to commercial power.
It is still another object of the invention to provide such a system in which means are provided to prevent such automatic closed transition when the alternate power source of the uninterruptible power supply is providing power to the load.
It is a subordinate object of the invention to effect the closed transition to commercial power from an uninterruptible power supply using maintenance bypass switches incorporating circuit breakers or molded case switches.